In general, the thermally sensitive recording medium which develops a recorded image using coloring reaction of colorless or pale colored basic leuco dye with a color developing agent caused by heat is actually applied for the facsimile use, for the computer field or for various kind of measuring instrument, because of it's advantages e.g. excellent clear coloring, noiseless at the recording process, relatively cheap price and easy maintenance. Still more, recently, additionally to the uses for a label or a ticket, the uses for an output medium for various printers or plotters such as a handy terminal for outdoor measurement or a delivering split, are rapidly increasing. In the case of above mentioned uses, since these uses are usually outdoor use, the quality and function which can resist against more serious atmospheric condition than the ordinary condition are required. That is, for example, resistance to water or humidity caused by rain, resistance to sunshine or high temperature condition e.g. inside of car at the summer season are necessary.
Regarding the improvement of water resistance, the use of a water soluble adhesive together with various crosslinking agent is disclosed in Japanese Patent Laid Open Publication 55-159993 and the use of water soluble adhesive having high crosslinking ability is disclosed in Japanese Patent Laid Open Publication 57-189889, however, these techniques are not sufficient. In the meanwhile, the method to improve the water resistance by use of the hydrophobic resin emulsion such as vinyl acetate emulsion, acrylic emulsion or SBR latex as an adhesive for the thermally sensitive recording layer is well known, however, since this method has problems to cause cohesion of head deposit or sticking, the actual use of it is not so popular. Further, in Japanese Patent Laid Open Publication 7-266711, the use of complex particle emulsion composed of self crosslinking acrylic emulsion and colloidal silica and/or colloidal silica and acrylic polymer or styrene acrylic polymer are proposed, however, since the product has the problem that the ground color of white part is deteriorated when the product is kept in high humid condition, the thermally sensitive recording medium which is sufficient to the actual use is not accomplished yet.